Stop checking for the response property, on XMLHttpRequest, when setting up the WorkerMessageHandler

This check was added in PR 2445, however it's no longer necessary since all data[1] is now loaded on the main-thread (and then transferred to the worker-thread).
Furthermore, by default the Fetch API is now (usually) used rather than `XMLHttpRequest`.

All in all, while these checks *were* necessary at one point that's no longer the case and they can thus be removed.

---
[1] This includes both the actual PDF data, as well as the CMap data.

src/core/worker.js

@@ -73,19 +73,7 @@ var WorkerMessageHandler = {
       // making sure postMessage transfers are working
       var supportTransfers = data[0] === 255;
       handler.postMessageTransfers = supportTransfers;
-      // check if the response property is supported by xhr
+
-      var xhr = new XMLHttpRequest();
-      var responseExists = 'response' in xhr;
-      // check if the property is actually implemented
-      try {
-        xhr.responseType; // eslint-disable-line no-unused-expressions
-      } catch (e) {
-        responseExists = false;
-      }
-      if (!responseExists) {
-        handler.send('test', false);
-        return;
-      }
       handler.send('test', {
         supportTypedArray: true,
         supportTransfers,